1. Field
This invention relates generally to charge pump, and more specifically, to a self-biased charge pump for a phase-locked loop.
2. Background
A phase-locked loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. The PLL is widely used in radio, telecommunications, computers and other electronic applications. They can be used to demodulate a signal, recover a signal from a noisy communication channel, generate a stable frequency at multiples of an input frequency, or distribute precisely timed clock pulses in digital logic circuits such as microprocessors.
The PLL may include a phase detector, a charge pump, a loop filter, a voltage-controlled oscillator (VCO), and a frequency divider. The VCO generates an output signal. The phase detector receives an input signal, compares the phase of the VCO-generated output signal with the phase of the input signal, and adjusts the VCO to keep the phases matched. The output of the phase detector also acts as a current source to pump current into and out of the loop filter by sending UP and DN signals to the charge pump to turn the charge pump on and off periodically. Since UP/DN current matching in a charge-pump is important to reduce noise and spur, the charge pump uses a replica bias branch for each of the UP circuit and the DN circuit. However, the replica bias branches add additional noise on the charge pump.